(1) Field of the Invention
This invention relates generally to the operation of disc recorders; and, more particularly, to controlling the speed and indexing of a recording disc.
(2) Description of the Prior Art
As is known, disc recorders include a rotating, disc-shaped magnetic medium in association with a transducer head for recording and reading information on the magnetic medium. Information can be organized on the disc in a plurality of circular tracks and further divided into a plurality of pie-shaped sectors. Accordingly, a given data group is indentifiable by the track and sector in which it is located. Locating a particular data group is typically done using electrical and mechanical means. Using purely electrical means typically requires an undesirably large amount of data storage space on the magnetic medium to be devoted to location-identifying information. On the other hand, using purely mechanical means typically requires undesirably stringent physical tolerances and stability of disc rotational speeds. Accordingly, the amount of electrical coding necessary to locate a particular data group is dependent upon the accuracy with which a given point on the disc can be located and the stability of the disc rotational speed. Typical electrical means include use of a preamble and a header containing electrical coding for identifying the data group location and for synchronizing internal clocks and decoding logic.
Speed control and indexing of recording discs using two axially-coupled slotted wheels rotating about an axis common to the recording disc is known. Typically a first slotted wheel with an associated first transducer are used for speed control and a second slotted wheel with an associated second transducer are used for indexing. The first slotted wheel has equally-spaced radial slots around the circumference which activate the first transducer to produce pulses whose repetition rate is indicative of the rotational speed of the recording disc. The second slotted wheel has a single slot to activate the second transducer to produce a pulse to indicate a given circumferential location on the recording disc.
Also, it is known to have a recording disc coupled to a slotted wheel having radial, circumferentially equally-spaced slots with one slot having a different shape to provide a reference point. For example, if one slot is longer than the others, it can be used to activate a first and a second transducer while all the other slots would activate only the first transducer. As before, the second transducer can provide a reference pulse to be used for indexing and the first transducer can provide a pulse repetition rate to be used for speed control. In both of the above-described techniques two tranducers are required. One transducer is required to detect the presence of the slots used to indicate rotational speed of the recording disc and another transducer is required to detect the slot used to indicate a reference point on the recording disc.